Electron-optical device for the recording of selected diffraction patterns

ABSTRACT

The direction of an illumination electron beam in an electron microscope is varied while a portion of the diffracted beam is projected in a predetermined fixed direction on the image plane and the remaining portions of the beam are intercepted.

Ilted States Patent van Dostrum ELECTRON-OPTICAL DEVICE FOR THERECORDING OF SELECTED DIFFRACTION PATTERNS Karel Jan van Oostrum,Eindhoven, Netherlands Assignee: U.S. Philips Corporation, New

York, NY.

Filed: June 14, 1974 Appl. No.: 479,468

Related US. Application Data Continuation of Ser. No. 348,062, April 5,1973, abandoned.

Inventor:

Foreign Application Priority Data Apr. 12, 1972 Netherlands 7204859 U.S.Cl. 250/307; 250/310; 250/397 Int. Cl. HOlJ 37/26 Field of Search250/310, 311, 396, 307,

References Cited UNITED STATES PATENTS 7/1961 Herrmann 250/397 1 Nov. 4,1975 3,180,986 4/1965 GribSOrl 250/397 3,626,184 12/1971 Crewe 250/3113,628,014 12/1971 Grubic 250/311 3,737,659 6/1973 Yanaka et a1 250/3113,833,811 9/1974 Koike 250/397 FOREIGN PATENTS OR APPLICATIONS 1,089,0889/1960 Germany 250/311 Primary ExaminerJames W. Lawrence AssistantExaminer-43. C. Anderson Attorney, Agent, or Firm-Frank R. Trifari;George B. Berka [57] ABSTRACT The direction of an illumination electronbeam in an electron microscope is varied while a portion of thediffracted beam is projected in a predetermined fixed direction on theimage plane and the remaining portions of the beam are intercepted.

8 Claims, 5 Drawing Figures VIIIIIIIIIIII U.S. Patent Nov. 4, 1975 Sheet2 of3 3,917,946

U.S. Patent Nov. 4, 1975 Sheet 3 of3 3,917,946

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ELECTRON-OPTICAL DEVICE FOR THE RECORDING E SELECTED DIFERACTIONPATTERNS This is a continuation of application Ser. No. 348,062, filedApr. 5, 1973, now abandoned.

The invention relates to a method of and a device for the generation andrecording of diffraction patterns of a comparatively small part of aspecimen to be selected.

In known methods, used in electron microscopes, a so-termed selectedarea diaphragm is used to select a part of a specimen which isirradiated by an electron beam for this purpose. As will be demonstratedherein after, the transverse dimensions of the part of the specimencontributing to the image formation cannot be chosen to be sufficientlysmall in all circumstances.

The invention has for its object to provide a method and a device inwhich a substantial reduction of the transverse dimensions of the partof the specimen to be separately examined is accompanied by increasedease of operation, also in the case of comparatively large diffractionangles.

A method of the kind set forth according to the invention ischaracterized in that the specimen is illuminated by an electron beam,the direction of incidence of which is varied during the recording of adiffraction pattern, a part of the electron beam which is diffracted ina fixed direction by the specimen being selected for the imaging of theobject in an image plane.

In order to form the image, the method according to the inventionutilizes only the electrons which move in a fixed direction afterdiffraction by the specimen. In a preferred embodiment, this directionis made to coincide with an optical axis of an electron-optical imagingdevice with the result that lens aberrations can exert only acomparatively small effect on the image formation. Using a diaphragm inthe electron-optical device, a limited directional region of electronsemerging from the specimen is thus selected. In a further preferredembodiment according to the invention, the variation of the direction ofincidence of the illumination electron beam is synchronized with thescanning of a scanning electron beam in a display unit. The outputsignal of a detector, selecting the part to be studied from the finalimage, is used to modulate the scanning beam in the display unit so asto record a diffraction pattern.

In the method according to the invention, the diffraction pattern isobtained from the detector as a signal which varies in time, so thatdigital data processing can be applied. Using a television camera tubefor the recording of a final image, the deflection of the illuminationelectron beam and that of the scanning electron beam are synchronizedwith respect to each other in the television camera tube in a preferredembodiment according to the invention. Given directional regions canthen be readily selected, and the display of an image can be realized incolour by the successive display of sub-images, each of the said colourscorre sponding to selected diffraction angles.

Known electron microscopes are often provided with a deflection unit forthe illumination electron beam. It is thus possible to use a microscopeof this kind for performing the method according to the invention,without essential structural modifications being required.

Some preferred embodiments according to the invention will be describedin detail hereinafter with reference to the drawings.

FIG. 1 is a diagrammatic view of a beam path in a known method.

FIG. 2 is a diagrammatic view of a beam path in a method according tothe invention,

FIG. 3 is a diagrammatic view of a beam path of an illumination electronbeam according to a preferred embodiment of the invention,

FIG. 4 is a diagrammatic view of a beam path of an illumination electronbeam for area scanning of the specimen, and

FIG. 5 is a block diagram of a preferred embodiment of a device forperforming the method according to the invention.

FIG. 1 shows a specimen plane 1, a diagrammatically representedobjective lens 2, a paraxial image plane 3, a zonal image plane 4, and aselected area diaphragm 5 of a known imaging system. An electron beam 6,forming part of the illumination beam, is incident on an object point Pin the specimen plane 1. The object in P diffracts part of the incidentelectron beam in preferred directions which are dependent of the objectand of the position of the object with respect to the incident beam 6.For the sake of simplicity, a non-diffracted beam 8, that is to say abeam extending along an optical axis 7 of the system, and an annularbeam 9 which is diffracted at an angle 0 and which originates from, forexample, a polycrystalline object, will be considered. The objectiveforms an image P in the paraxial image plane 3 of the object point P inthe non-diffracted beam 8. In the diffracted annular beam 9, theobjective lens images the point P in a point P of the zonal image plane4. The zonal image plane does not coincide with the paraxial image plane3 as a result of spherical aberration of the objective lens which is afunction of the angle 6. In the paraxial image plane, the beam 9 forms aring about the point P. The selected area diaphragm 5, corresponding toa part of the object in the specimen plane, will allow or will not allowpassage of this ring, depending on the angle 6.

Electrons which are diffracted in a corresponding manner by an objectpoint Q in the specimen plane, can also arrive at P via a zonal imagepoint Q" as is denoted in the Figure by a broken line 10. Thediffraction pattern produced on a target by the electron microscope,consequently, not only originates from a disc about the object point Pwhich is paraxially associated with the selected area diaphragm, butalso from an object point O which is situated outside this disc. At acommonly used diffraction angle 0 of 0.1 rad., selection of a part ofthe specimen which is dimensioned smaller than approximately 1 micron bymeans of a selected area diaphragm is not possible according to thismethod.

Corresponding elements in FIG. 2 are denoted by the digits and lettersused in FIG. 1. The direction of incidence at the object point P of apart 12 of the illumination electron beam which is directed at P iscontinuously varied during the recording of a diffraction pattern.Expressed in spherical co-ordinates 0 and (b, 6 being given by the anglebetween the main beam of the electron beam incident in P and the opticalaxis 7 of the system, the variation in the'direction of incidence can berealized, for example, in a pattern at which the angle (1) completes a360-trajectory, each time at a fixed value of 0. A spiral-like movementcan also be chosen, the angle 0 then being changed comparativelyslightly at each 360-trajectory for d). the illumination beam can alsobe deflected according to a normal television frame pattern. For thedisplay of sub-images in selected illumination as described in thepreamble, the current intensity in the beam can be controlled, forexample, such that illumination takes place only at preselecteddirections of incidence. Therefore, the current intensity control ispreferably binary, but selected directions can alternatively beilluminated with a less intensive electron beam than other directions.For the same purpose, discrimination as regards given preferreddirections can also take place during recording. A part 13 of the beam12 is diffracted in a direction along the optical axis 7. Thediffractionangle is again denoted by 6 in FIG. 2. Using electrons moving along theoptical axis 7, the objective lens forms an image P of P in the paraxialimage plane 3. Non-diffracted electrons which form a beam 14 areintercepted by a diaphragm 15 which is arranged in the image focal planeof the objective.

In fact, this angle selection constitutes dark-field illumination in adirection which varies in time. The diaphragm 15 allows passage only ofelectrons which are diffracted in the object point P at an angle 6.Electrons from an object point Q cannot reach the image point P.Regardless of the direction of incidence, the point P is reached only byelectrons which paraxially pass through the electronoptical systembehind the specimen plane. If the direction of incidence of beam 12 isvaried, the intensity of the electron beam incident in P will be varied.This is because the diffracted fraction of the beam 12 is a function ofthe directional coordinates d) and 6. The nautre of this function isdependent of the nature of the object in P. Because only paraxial beamscontribute to the image formation, the effect of aberrations oftheobjective on the image formation will be comparatively small. The objectdisc about P, now corresponding to a sensitive surface of a detector 16in the image plane, can also be substantially smaller than in the knownmethod. with commonly used objective lenses, the transverse dimensionsof the object disc can be less than approximately 10 A. By means of thedetector 16, the intensity of the electron beam incident in the imagepoint P is measured as a signal which varies in time.

A preferred embodiment according to the invention for the control of thedirection of the illumination beam as shown in FIG. 3 comprises anelectron-optical lens system 17 for which any commonly used objectivelens can be used. A part AB, the field of vision, of a specimen isimaged, in a sectional view, as A'B in the image plane 3. To this end, across-over of the electron beam in a point Q is imaged, by means of acondenser part 19 of the lens system 17, in a virtual point 18 fromwhich the field of vision AB is illuminated. The cross-over in Q isformed in known manner by a preceding condenser system (not shown). Bymeans of a deflection system 20, the point Q is shifted in a horizontalplane, i.e. transverse to the optical axis 7 of the system. By means ofthe deflection system 20 the illumination electron beam is tilted abouta virtual tilting point R, As a result, the angle 6 at which the fieldof vision is illuminated is varied, without the field of vision itselfbeing varied. To this end, the point R is chosen as the tilting point,this point being virtually associated with a centre point C of the fieldof vision AB by the condenser lens 19. Consequently, C is the realtilting point. By means of the diaphragm 15, the image-formation by anobjective lens 21 is limited to the electrons which are diffract ed bythe specimen along the optical axis 7. Electrons originating, forexample, from a virtual point 22 and which pass the object without beingdiffracted, are intercepted by the diaphragm 15. The foregoing isindicated by means of an electron beam 23 which is denoted by a brokenline. The point 18 can be displaced in a plane 25, for example, betweenthe points 22 and 23 by means of the deflection system 20. This movementcan be performed, for example, in a pattern of concentrical circlesabout the axis 7 of the system. The diffraction direction can be readilyselected. The illumination of an object at a varying angle of incidence6 and azimuth d) is thus realized by displacement of a cross-over of theillumination electron beam in a plane transverse to the optical axis.

For the sake of comparison, FIG. 4 shows a beam path for the areascanning of a specimen. An electron beam 26 is focussed in a virtualpoint 27 by a condenser system not shown, and is deflected by means of adeflection system 28, a point 29 serving as the tilting point. Acondenser part 30 of the lens 17, in this case formed by a Ruskacondenser objective lens, see Riecke and Ruska, Proceedings VIth,Intern. Congress for Electron Microscopy, Kyoto, Japan 1966, Volume I,page 19, forms, on the basis of the virtual cross-over in 27, a realpoint 31 in the specimen plane 1 which is situated in the centre in thislens. By moving the electron beam 26 about the tilting point 29, areascanning of a specimen takes place. For the tilting point 29 the focalpoint of the condenser lens 30 is chosen such that any point of thespecimen is illuminated at the same direction of incidence. An angle ofincidence of is shown. An objective lens 32 of the Ruska lens 17 forms adiffraction pattern in the image focal plane 33 thereof, the saidpattern thus retaining its place when the electron beam 26 is moved inthe specimen plane. In the image focal plane or in an enlarged imageformed by subsequent lenses which are not shown, the diffraction patterncan be measured, for example, by means of a ring detector.

The change-over from directional scanning according to the invention toarea scanning can be realized in one and the same lens system byadaptation of the beam path of the illumination electron beam. It isthen necessary to choose a correct position of the tilting point and thecross-over of the beam. This can be realized by means of suitableadjustments of the various electron-optical lenses and deflectionsystems. In an electron microscope such as, for example, the Philips EM300, the method according to the invention can be performed withoutmajor structural modifications being necessary. This is because anelectron microscope of this kind is provided with a wobbler (deflectionsystem) having a range of approximately 0.1 rad. in 6. A range of thiskind is more than sufficient for performing the method according to theinvention. Besides the possibility of examining a smaller part of thespecimen, there is an additional advantage in that the case of operationis enhanced. A final image a diffraction pattern can be studied at thesame lens adjustments according to the invention. Using the methodaccording to the invention, a recordable final image is successivelyobtained for different illumination directions, in which a desiredsub-image can be readily selected in the image plane by means of adetector so as to record the diffraction pattern of the correspondingpart of the specimen.

Another method of making a diffraction pattern of a small part of aspecimen is described in the article Beugungsexperimente mit sehr feinenElektronen- 5 strahlen" in Zeitschrift Angew. Phys. 27, No. 3, 1969,pages 155-165. Therein, use is made of a condenser objective lens asdescribed with reference to FIG. 4. The selection of the part to bestudied is not effected by means of a diaphragm in the paraxial imageplane of the objective, but by limitation of the illumination to theselected part of the specimen. Because the condenser part of the lensused for this purpose is very strong and has little sphericalaberration, the illumination beam on the specimen can have a smallcross-section, so this method also enables selection of a comparativelysmall part of the specimen. In contract with this method, in the methodaccording to the invention the minimum achievable dimensions of theselected part are not dependent of aberrations in the deflection andcondenser system, and hence they are independent of the use of thespecial lens with a strong condenser field.

A method of imaging a specimen in selected dark field illumination isdescribed in the article selected zone dark-field Electron Microscopy,Appl. Phys. Lett. 20, No. 3, 1-2-1972, pages 122-125. Therein, thediffraction angle 6 is selected by using an annular diaphragm in theimage focal plane of the objective lens. According to this method, afinal image of an object is produced by means of electrons which arediffracted inside a directional region selected by the diaphragm. Theadjustment of such an annular diaphragm is difficult. Moreover, adifferent diaphragm must be used for each diffraction direction.

The block diagram of FIG. 5 shows an electron gun 40 for generating theillumination electron beam, two sets of deflection coils 41 and 42 fordeflecting the electron beam, each set being active in two directionswhich are perpendicular to each other, with the result that deflectionin all directions and d) with respect to an axis 47 is possible, aspecimen plane 43, an objective 44, and a target 45. All said elementsare components of a commonly used electron microscope such as the saidPhilips EM 300. Coupled to the target 45 is a detector 46 for detectingthe intensity of the electron beam. The diagram also shows an electroniccontrol circuit 48, an excitation unit 49 for the deflection coils, asignal converter 50, and an x-y display unit 51. A signal derived fromthe detector 46 is applied to the signal converter 50. The signalconverter converts the signal which is analogous to the measuredintensity into a modulation signal which is applied to the x-y displayunit as z-modulation. Like the x-y display unit, the signal converter iscoupled to the control circuit 48. Via the excitation unit 49, thecontrol circuit controls the deflection of the illumination beam insynchronism with a scanning beam in the x-y display unit. Thediffraction pattern of the part of the specimen which corresponds to thedetector surface is displayed on the screen of the display unit. If theelectron microscope is provided with a television camera tube for imagerecording, the part of the video signal which orignates from theselected region in the image plane can be used as the detector signal.

In a preferred embodiment according to the invention, the electron gun40 is controlled by the control circuit such that an electron beam isproduced only during a desired or pre-set sequence of directions ofincidence, measured according to coordinates 6 and d). A final image,recorded by a television camera tube 52, then only containing structureshaving a selected diffraction pattern. The detector is equipped with aluminescent screen to which the television camera tube is connected,preferably by means of a fibre-optical win dow. A similar selection canbe performed by recording the video signal, at a continuous variation inthe direction of incidence, only when the illumination beam is in aselected directional region. By alternately selecting a number ofmeasuring sequences and by assigning a colour to each of these sequencesin accordance with known colour television techniques by means of achromatizing unit 53, an image can be displayed on a colour monitor 54in which each colour is characteristic of a specimen structure havingits own diffraction angle. So as to achieve proper synchronization, itis ob vious that the chromatizing unit must be coupled to the controlcircuit. In the case of a scanning pattern given by circles fordifferent values of 0, the zero-order diffraction can be readily avoidedin the colour television signal and can, if necessary, be separatelydisplayed on a black-white monitor. It is obvious from the describedblock diagram that the method can be performed on any electronmicroscope provided with a Wobbler unit for deflecting the electron beamaccording to television techniques. A device for performing the methodaccording to the invention can be constructed by addition of knownelectronic circuits and systems for recording and displaying imageswhich are known from the television technique.

What is claimed is:

l. A method of generating and selectably recording diffraction patternsof a specimen in an electron microscope, comprising the steps ofdirecting an illumination electron beam in a first direction at asubstantially fixed angle to said specimen, wherein said specimen islocated at the intersection of a specimen plane with the optical axis ofthe microscope and said illumination beam is inclined at an acute angleto said axis, projecting in a fixed second direction a portion of thebeam diffracted from the specimen, on an image plane in which saidsecond direction coincides substantially with said axis, interceptingremaining portions of the beam, and varying said first direction of theillumination beam while recording the projected diffraction pattern insaid image plane.

2. A method as claimed in claim 1, comprising the steps of arranging adetector in the image plane for recording a time-dependent signal for adiffraction pattern of a part of the specimen which is selected.

3. A method as claimed in claim 2 wherein said detector is a televisioncamera.

4. A method as claimed in claim 3, wherein said illumination electronbeam is deflected in synchronism with the recording signal of saiddetector.

5. A method as claimed in claim 4, wherein said illumination electronbeam is deflected by a control unit by means of an electron-opticaldeflection system, in accordance with a television frame pattern, andcontrols in synchronism therewith a recording device, said methodfurther comprising the step of recording a signal only at preset firstdirections of incidence of said illumination electron beam.

6. A method as claimed in claim 5 wherein said detector is a colortelevision camera.

7. A method as claimed in claim 1, wherein said vari ation in said firstdirection of incidence is realized in spherical coordinates 0 and d) andover a trajectory of 360 at different, substantially fixed values of theangle 0 and the angle :1).

8. A method as claimed in claim 7, that the angle 0 is varied in steps,the angle (1) varying over 360 between

1. A method of generating and selectably recording diffraction patternsof a specimen in an electron microscope, comprising the steps ofdirecting an illumination electron beam in a first direction at asubstantially fixed angle to said specimen, wherein said specimen islocated at the intersection of a specimen plane with the optical axis ofthe microscope and said illumination beam is inclined at an acute angleto said axis, projecting in a fixed second direction a portion of thebeam diffracted from the specimen, on an image plane in which saidsecond direction coincides substantially with said axis, interceptingremaining portions of the beam, and varying said first direction of theillumination beam while recording the projected diffraction pattern insaid image plane.
 2. A method as claimed in claim 1, comprising thesteps of arranging a detector in the image plane for recording atime-dependent signal for a diffraction pattern of a part of thespecimen which is selected.
 3. A method as claimed in claim 2 whereinsaid detector is a television camera.
 4. A method as claimed in claim 3,wherein said illumination electron beam is deflected in synchronism withthe recording signal of said detector.
 5. A method as claimed in claim4, wherein said illumination electron beam is deflected by a controlunit by means of an electron-optical deflection system, in accordancewith a television frame pattern, and controls in synchronism therewith arecording device, said method further comprising the step of recording asignal only at preset first directions of incidence of said illuminationelectron beam.
 6. A method as claimed in claim 5 wherein said detectoris a color television camera.
 7. A method as claimed in claim 1, whereinsaid variation in said first direction of incidence is realized inspherical coordinates theta and phi and over a trajectory of 360* atdifferent, substantially fixed values of the angle theta and the anglephi .
 8. A method as claimed in claim 7, that the angle theta is variedin steps, the angle phi varying over 360* between successive steps intheta .